Bathymetric survey at hydrothermal site by L-array Interferometric sonar mounted on AUV-r2D4

Autonomous underwater vehicle is expected for its stability as platform of many sensors. AUV-r2D4 that can act under the sea up to a depth of 4,000 m was built for the purpose of finding and surveying hydrothermal deposit and already with many advanced sensors some investigations have been performed. For r2D4 we have been developing new bathymetric sonar that arranges three hydrophones in L-array shape beside both side's transmitters usually used as side scan sonar. These systems calculating the angles of arrival of backscatter signals from differential phase called interferometric or bathymetric side scan consist of two hydrophones and one transmitter. Our new system arranged three hydrophones at L-array shape (LA-IFS) enabled us to resolve unwrapping problem and led us to make higher resolution bathymetric chart. LA-IFS has advantages for AUV because of its simple design and energy saving performance. And as bathymeter its long range and dense swath bathymetry, of course backscattering, provides effective alternative to AUV equipment. We have performed some investigation with r2D4. In this paper we introduce the results of LA-IFS's bathymetric surveying at the Central Indian Ridge on December 2006.